1. Field of the Invention
The present invention relates to a backlight unit providing illumination for a liquid crystal display.
2. Description of Related Art
Recently, liquid crystal display devices have been remarkably improved, and their market has been steadily growing because they can save installation space by virtue of their thickness, and they can also save power.
Prior art liquid crystal display devices comprise a liquid crystal panel including a liquid crystal layer interposed between a pair of glass substrates, and two polarizing plates, which are attached on opposite sides of the glass substrates. The polarization state of linearly-polarized light incident upon the liquid crystal panel is modulated by the liquid crystal layer, whereby images are displayed.
A transmission-type liquid crystal display device, which is currently the dominating device on the market, requires illumination from its rear. Therefore, a backlight unit is an essential element for a transmission-type liquid crystal display device.
A conventional backlight unit 20, as shown in FIG. 4, consists of a light source 1, a light guide plate 2 having a first end surface 21 located in the vicinity of the light source 1, a diffusing plate 3 arranged on a top surface of the light guide plate 2 and a reflective plate 4 disposed on a bottom surface 23 of the light guide plate 2. A plurality of reflection dots 24 are disposed on the bottom surface 23 of the light guide plate 2. The light guide plate 2 also comprises a second end surface 22 opposite to the first end surface 21 and a top surface 25 opposite to the bottom surface 23. The conventional backlight unit 20 operates in the following manner: first, light beams emitted from the light source 1 enter into the light guide plate 2 through the first end surface 21 thereof, and are then transmitted to the second end surface 22 after being sequentially reflected by the top surface 25 and the bottom surface 23 (reflecting surface). During the transmission, most of the light beams emit from the top surface 25 and a few of the light beams emit from the bottom surface 23 and are reflected by the reflective plate 4 back into the light guide plate 2. The light beams emitting from the top surface 25 enter into the diffusing plate 3 and are diffused by the diffusing plate 3. Then the light beams emerge from the diffusing plate 3 so that the backlight unit 20 is usable as a surface light source.
However, such a backlight unit 20 using the light guide plate 2 having the reflection dots 24 on the bottom surface 23 is subject to some problems. First, because printing the reflection dots 24 on the light guide plate 2 involves use of water, which is absorbed by the light guide plate 2, and the light reflection dots 24 are only printed on one side of the light guide plate 2, the light guide plate 2 will change its shape due to water absorption on only one side. Thus, the light guide plate 2 needs to be dried after the reflection dots 24 are printed on its bottom surface 23. This results in an increase in the cost of the backlight unit 20. Second, the light guide plate 2 may be damaged during the course of printing the reflection dots 24 on the light guide plate 2, which also increases the cost of the backlight unit 20.